The stabilization of soils containing water-expansive clay minerals (chiefly montmorillonite) by treating such soils with lime is well established. Stabilizing reactions of several types occur, with ion-exchange and agglomeration/flocculation reactions resulting in a reduction of plasticity of the clay soil and, over a more extended period, with a pozzolanic soil-lime reaction often resulting in the development of cementitious products of increased strength and moisture-barrier properties. Ho, C., and R. L. Handy, Special Report, 42nd Annual Meeting of Highway Research Board, NAS-NRC, January 1963; Lyons, J. W., Engineering News-Record, 101-105, Aug. 15, 1957. The goals of such treatment are to increase the soil strength and reduce the volume changes that occur in relation to the moisture content of such expansive soils and thereby limit the structural damage caused by such changes. It has been estimated that the swelling and shrinking of soils causes billions of dollars in damage to houses, commercial buildings, roads, and pipelines---considerably more than the damage caused by floods, hurricanes, tornadoes, and earthquakes. Jones, D. E., Civil Engineering--ASCE, 49-51, August 1973; Godfrey, K. A., Civil Engineering--ASCE, 87-91, October 1978.
Various techniques have been offered for treating moisture-expansive soils with lime to stabilize them and control swelling. One such procedure involves mechanically mixing the soil of a building site with hydrated lime, either by mixing the lime into the soil in situ by equipment similar to that used for cultivation, or by removing a layer of soil, mixing it with lime in a suitable mixer, and then replacing the lime-containing soil. Another technique involves first ponding the foundation subsoil to pre-expand it, followed by lime-stabilizing of the top 12-inch layer. Gutschick, K. A., paper presented at NLA Convention, Point Clear, Ala., May 12, 1964. A modified ponding technique has also been suggested consisting of first forming a maze of cross trenches 6 inches wide and 3 feet deep, on 10 foot centers in one direction and 30 foot centers in the other, and then filling such trenches with hydrated lime to a depth of 12 inches, followed by back-filling with washed pea gravel. The trenches are then flooded, using enough water to keep them constantly wet but not overflowing. Moisture content is checked periodically to a substantial depth (10 feet) about the site area, the treatment being considered complete in about 2 to 3 weeks when the moisture content reaches approximately 30 percent.
While such techniques have met with varying degrees of success for preparing a site prior to the laying of a foundation, they are less suitable or effective for treating a site on which a building foundation is already located. The soil about such a foundation may be removed and replaced with compacted non-expansive soil or with chemically-treated (i.e., lime-treated) on-site soil. Holtz, W. G., and S. S. Hart, Home Construction on Shrinking and Swelling Soils, October 1978. Such a procedure may produce a cementitious mass that reduces entry of water, but substantial expansive forces tending to cause damage to basement walls, and particularly upward forces tending to heave a floor slab (whether below-grade or on-grade) will continue to develop.
U.S. Pat. No. 3,274,783 discloses a method for stopping landslides and soil shear failures by forming bore holes about the periphery of a foundation and then introducing hydrated lime into such holes to at least the top of the perched water table-containing soil stratum beneath the foundation. The hydrated lime increases the mechanical strength of the saturated montmorillonitic clay strata, thereby preventing soil shear failure. While such treatment has been found effective in preventing or stopping landslides, it does not significantly reduce, and may even increase, the stresses that tend to cause damage to foundations in moisture-expansive soils. Similarly, shallow trenching about a foundation, even where such a trench is used to connect the top portions of bore holes formed as disclosed in such patent, and then filling the trench with hydrated lime, have been found ineffective in preventing or appreciably reducing foundation damage, especially damage caused by uplifting of below-grade or on-grade floor slabs.
This invention is therefore concerned with a method for treating the moisture-expansive soil about a pre-existing foundation to relieve stresses that might otherwise develop and damage the foundation, particularly those stresses tending to cause an uplifting and cracking of basement floor slabs and on-grade floor slabs. It is a specific object of this invention to provide a method that not only is effective in controlling expansive clay soil beneath and about such a foundation to avoid or at least greatly reduce the likelihood of such damage, but to do so by a sequence of relatively uncomplicated and inexpensive procedural steps.
One aspect of this invention lies in recognizing that the upward forces exerted by swelling clay soil beneath a floor slab may be relieved by trenching about the foundation to a depth substantially below that slab and then substantially filling the trench with hydrated lime to provide a compliant lime buffer zone about the foundation capable of accommodating lateral expansion of the soil beneath the slab and thereby reducing the uplifting forces exerted upon the slab. Such buffer zone also cushions or absorbs the horizontal forces directed towards the side walls of a basement foundation while, at the same time, allowing lime to diffuse into the soil about and beneath the foundation to cause stabilization of that soil.
Briefly, the method includes the steps of first trenching along at least one side of the concrete foundation of a building structure, and usually about the entire perimeter of that foundation, to a depth well below the level of the concrete floor slab of that foundation, and then substantially filling the trench with a flowable plastic mass of hydrated lime and water. The vertically-displaceable contents of the trench relieve lateral stresses caused by expansion of the surrounding subsoil and reduce the upward forces generated by the absorbent clay beneath the foundation slab. In addition, the hydrated lime slowly diffuses into and stabilizes the surrounding subsoil to reduce soil expansion and plasticity.
The depth of the lime-containing trench should be at least 6 inches below the floor slab of the foundation and extend below any footing that may be provided as part of that foundation. Greater depths are preferred because of the greater capacity of the lime buffer or cushion to accommodate lateral expansion of the clay beneath the slab. The widths of the trench should be within the range of about 2 to 12 inches, preferably 3 to 5 inches, and be spaced from the periphery of the foundation a distance up to about 8 feet, preferably no more than about 3 feet. Along those sides of the foundation facing uphill slopes, parallel vertical trenches spaced 2 to 8 feet apart, preferably about 3 to 5 feet apart, may be provided to accommodate lateral creep of the soil and to promote greater stabilization of the soil between the parallel trenches.